Monitoring and measuring apparatus for frequency modulated signals



Feb. s, 1945.

R. A. BRADEN MONITORINGAND MEASURING APPARATUS FOR FREQUENCY MODULATED S IGNALS Filed Feb. 27, 1942 E. 6/6/VJL v MODKl/LT /z 'l WA Gttorncg l Patented Feb. 6, 1945 MONITORING AND MaAsUaING APPARATUS Foa FREQUENCY MQDULATED sIGNALs Rene A. Braden, Haddon Heights, N. J., assigner Ato Radio 'Corporation of America, a corporation of Delaware Application February 27, 1942, Serial No. 432,632; 4 anims.' (ci. 17e-111.5).

'I'his invention relates'generally to electrical measuring apparatus, and particularlyto monitoring and' measuring apparatus for frequency .'modulated signals in which observation of the 'peak frequency swing of such signals is \of interest.

Heretofore various methods and apparatus for measuring the peak frequency swing of frequency modulated signals have been used, but all have involved the use of -complicated and carefully adjusted circuits for selecting the particular lfreous frequency values of the signals.

lIt is an object of this invention to provide a method and apparatus for utilizing conventional means for deriving the instantaneous frequency deviation of the frequency modulated signals with reference to a mean. or reference carrier fre-. quency source, combined with a new and simple l method and apparatus for indicating the value or degree of this instantaneous frequency deviation. It is another object of the invent-ion to provide amethod and apparatus for amplitudev modulating the frequency modulated signal with a reference carrier frequency to derive a beat frequency corresponding to the mean deviation between the frequency modulated signal `and the vreference frequency, in which the beat frequency is applied to a measuring instrument as a function of the modulating component -oi the frequency modulated signals.- .It is en additional object of the invention to provide a method andA apparatus for selecting frequency limits and indicating overmodulation', with respect to such limits, in a frequency modulated signal.

The invention will be described byreference lto the drawing in which Figure l is a schematic circuit diagram of one embodiment of the invention; FigureA 2 is a schematic circuit diagram of another embodiment of the invention, and-Figure 3 is a schematic block circuit diagram of still another embodiment of the invention.

Referringto Flgurel.. of the drawing, a'sourceA of frequency modulated'signals is connected to the input circuit I of amodulator 2. A source control electrodeo'f apentode 4type electron dis-.

charge tube Lhaving high anode resistance. 4A 9relatively high inductance B Ais included in the quency bands required for observation and forV providing suitable indications of the instantane.

veloped across the anode circuit of tube 5` is'applied to the vertical deflecting element 1 of a vcathode ray oscillograph tube i3. Since the anode load of -tube 5' is a substantially pure inductance the voltage developed across the anode circuit will be directly proportional to the frequency applied to the control electrode. Therefore, the vertical deection of the cathode ray ofthe oscilloscope l3- will be directly proportional to frequency. vAsou'rce of signals corresponding 4to the modulating component of the frequency modulatedfsignal under observation is connected to the terminals 8 of a variable phase shifter 9. 'I'he output'of thevphase shifting'circuit is connected to the. horizontal' deecting'elements I0 of thel cathoderay oscillograph-IS. This horizontal deflecting signal provides a .timingvoltage which permits the instantaneous frequency deviation of the frequency modulated signal to beobserved. The signals corresponding to the modulating component of the frequency modulated signal may be derived either directly from the frequency modulated signal inputI circuit-or lthey may be derived 4from the transmitted signal under observation vby means of a conventional fre-l quency modulation receiver.

Referring to Figure 2 of the drawing, a frequency modulated "signal to be observed is appliedtothe terminals of'a modulator 2l.v A reference carrier frequency, preferably Icorresponding to the mean carrier frequency of .the frequency modulated signals is applied tothe terminals of the modulator circuit 2|. The output of the modulator -`circuit 2| is connected=to the input of.a rst variable gain ampller 2 3 and is also connected to the input of a second variaanode circuit of the tube l. The voltage deu ble gain amplifier 21. 'The output ofthe :first amplifier 23 is connected tothe control 'electrode of a pentode thermlonic Itube 2l having high anode resistance. A substantially pureinductive load 25 is connected 'in the yanode vcircuit of tube 2l. The output ofthe :second amplifier 21. is :connected to the control electrode of a pentode thermionic tube, having. high anode resistance. AV substantially .pure inductive 'load' 29 is 'connected in' the sanode circuit .of thel .tube 2l. Switching means 22 are provided for Aconnecting a peak reading voltmeter M across the anode clrcuits of either offthe tubes 2l 01"28, as desired. Grid bias for the variable gain amplifiers 2! and 21 is obtained from a square wave limiter-ampli- -fler 33 which is actuated through a ,balancedcircult 2| by signals'corresponding to the modulating component of the frequency modulated signals .under observation. This modulating component l wave pulses.

can 'be obtained directly from the modulation input circuit of the frequency modulated signal source, or by means of a conventional frequency modulation receiver actuated by the frequency modulated signals. The output of the square wave amplifier is used to trigger the Variable gain amplier 23 on positive square wave pulses and the variable gain amplifier 21 on negative square Since the duration of the square wave pulses is considerably greater than the d'uration of the frequency swing of the frequency modulated signal, no phase shifting network is required for satisfactory timing. The amplifier 23 'will conduct during periods of. positive frequency deviation, and the degree f deviation Will be indicated by the voltage, proportional to frequency, developedacross the inductive load 2-5, as previously explained. The ampliiier 21 will become conducting on negative frequency deviation and the degree of deviation will be indicated by the voltage proportional to frequency developed acrossthe inductive load 29. I 1

. Figure of the drawing provides a method and circuit for indicating over-modulation. A source of frequency modulated signals under observation is connected to the terminals 40 which are connected to the input circuits of modulators 4| and 43. A variable frequency oscillator 42 is connected to the input of the modulator 4l and av second variable frequency oscillator 44 is connected to the input of the modulator 43. 'I'he frequency of the oscillator 42 is adjusted slightly above the desired maximum frequency limit of the frequency modulated signal. The frequency of the oscillator 44 is adjusted slightly below the minimum desired frequency limit of the frequency modulated signal. The output of the modulator 4| is connected through a conventional low pass lter 45 to an indicator 41. The output of the modulator 43 is connected through a low-pass filter 46'to an indicator 48. It will be'apparent that if the frequency modulated signal under ob- `servation swings to or through values which produce beat frequency currents in the output of the modulators which will be passed by the low-pass lters, either one or the other of the indicators will be actuated. It will be apparent that the frequency of oscillators v42 and 44 can be adjusted to indicate overmodulation in either symmetrical for unsymmetrical frequency modulated signals.

It will be understood that the circuit components illustrated in) the drawing by block dia,-

gram, and not described in detail, 'are of con,

Well known to those skilled in rive currents of lfrequency corresponding to the instantaneous frequency difference between said signals and said carrier, means for applying said currents to the control electrode of said tube, means for applying the voltage developed across said tube anode load circuit to one pair ofdeiecting elements of said oscillograph, and means for 'applying the frequency modulating component of said frequency modulated signals to the other pair of deflecting elements of said oscillograph. i

2. Apparatus for observing the peak frequency swing of frequency modulated signals including in combination a source of frequency modulated signals,A a cathode ray oscillograph having pairs of deilecting elements, a vreference carrier frequency source, an electron discharge tube having relatively high anode resistance and a substantially pure inductive anode load circuit, means for mixingv said signals and said carrier to derive currents of frequency corresponding to the instantaneous frequency difference between said signals and said carrier, means for applying said currents to the control electrode of said tube, means for applying the. voltage developed across said tube anode load circuit to onepair of said deflecting elements, and means for applying the frequency modulating component of said frequency modulated' signalsvin desired phase relation to said voltage to the other pair of said deecting elements.

3. Apparatus for observing the peak frequencyt swing of frequency modulated signals including in combination a source of said signals, a reference carrier frequency source, means for mixing said signals and said carrier, a rst amplifier for the modulation component of said mixed signals higherin frequency than the carrier, 'a second amplifier for the modulation component .of said mixed signals lower in frequency than' the carrler, a, substantially purely reactivev load circuit connected to the output circuit of each vof said amplifiers, an indicator connected to each of said load circuits, means for deriving square wave voltages as a function of the modulating component of said signals, and means for varying the gain 0f said amplifiers by said square wave voltages.

4. Apparatus for observing the peak frequency swing of frequency modulated signals including Y f'cuit means to said indicator as a function of the frequency modulating component of said signals.

RENE A. BRADEN. 

